Parity-consistent filling of a free-fermion band
= Parity-consistent filling of a free-fermion band
When a quadratic <fermion> spectrum was derived in a fixed <fermion parity> sector, filling all negative-energy modes is permitted only if the resulting particle number has that parity. If it does not, toggle the occupation of a mode with smallest absolute <energy>. Compare the minimum energies of the physical sectors. Zero-energy modes can create degeneracy without <energy> cost.